Our objective was to examine the induction of endothelial cell apoptosis by the proinflammatory ligand, TNFα. We hypothesized that TNFα influences endothelial cell viability by altering the balance of regulatory molecules that either induce or suppress apoptosis. Since the identity of these regulators is unknown, our approach was to examine induction of endothelial cell apoptosis by TNFα alone and in the context of an inhibitor of transcription or translation. TNFα was able to induce bovine pulmonary artery endothelial cell apoptosis in a dose-dependent fashion. Inhibition of transcription with actinomycin D or translation with cycloheximide also resulted in apoptosis, which reached a maximum value of approximately 35 to 40% of cells after 24 h. TNFα induction of apoptosis was either potentiated or abrogated by cycloheximide, depending on the timing of TNFα exposure in relation to inhibition of protein synthesis. When cycloheximide was added concomitantly with midrange concentrations of TNFα, there was both a dramatic acceleration and a synergistic increase in the observed apoptotic response, with all endothelial cells dying within 24 h. When cycloheximide was added as a function of time after termination of TNFα exposure, the synergistic induction of apoptosis was maintained at > 70% of its maximum value for 1 h, declining monotonically in a time-dependent fashion to baseline values after 6 h. In contrast, when TNFα was added after protein synthesis was inhibited, no additional increase in apoptosis above that observed with inhibition of protein synthesis alone was observed. Our results are consistent with the concept that endothelial cell viability depends on an interaction of inducers and suppressors of apoptosis, which are susceptible to modulation by TNFα.